1. Technical Field of the Invention
The present invention is directed towards downstream treatment and purification of olefins generated by dehydrogenation of alkanes. More particularly, the present invention is directed toward a process for generating alcohols from alkanes by dehydrogenating one or more alkanes to olefins followed by hydrating the olefins to alcohols.
2. Description of Related Art
There is currently a significant interest in various types of hydrocarbon processing reactions. One such class of reactions involves the chemical conversion of natural gas, a relatively low value reactant, to higher value products. Natural gas comprises several components, including alkanes. Alkanes are saturated hydrocarbons—i.e., compounds consisting of hydrogen (H) and carbon (C)—whose molecules contain carbon atoms linked together by single bonds. The principal alkane in natural gas is methane; however, significant quantities of longer-chain alkanes such as ethane (CH3CH3), propane (CH3CH2CH3), and butane (CH3CH2CH2CH3) may also be present. Unlike longer-chain alkanes, these short-chain alkanes are gaseous under ambient conditions.
The interest in the chemical conversion of the methane and higher alkanes in natural gas stems from a variety of factors. First, vast reserves of natural gas have been found in remote areas where no local market exists. There is great incentive to exploit these natural gas formations because natural gas is predicted to outlast liquid oil reserves by a significant margin. Unfortunately, though, the transportation costs for gaseous alkanes are generally high, primarily because of the extremely low temperatures needed to liquefy these highly volatile gases for transport. Consequently, there is considerable interest in techniques for converting gaseous alkanes to higher value, more easily transported products at the remote site.
Several hydrocarbon processing techniques are currently being investigated for the chemical conversion of lower alkanes. One such technique involves the conversion of methane to higher chain-length alkanes that are liquid or solid at room temperature. This conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is partially oxidized to produce a mixture of carbon monoxide and hydrogen known as synthesis gas or syngas. In a second step, the syngas is converted to liquid and solid hydrocarbons using the Fischer-Tropsch process. This method allows the conversion of synthesis gas into liquid hydrocarbon fuels and solid hydrocarbon waxes. The high molecular weight waxes thus produced provide an ideal feedstock for hydrocracking, which ultimately yields high quality jet fuel and superior high cetane value diesel fuel blending components.
Another important class of hydrocarbon processing reactions are dehydrogenation reactions. In a dehydrogenation process, alkanes can be dehydrogenated to produce alkenes. Alkenes, also commonly called olefins, are unsaturated hydrocarbons whose molecules contain one or more pairs of carbon atoms linked together by a double bond. Generally, olefin molecules are represented by the chemical formula R′CH═CHR, where C is a carbon atom, H is a hydrogen atom, and R and R′ are each an atom or a pendant molecular group of varying composition. One example of a dehydrogenation reaction is the conversion of ethane to ethylene [1]:C2H6+Heat→C2H4+H2  [1].The non-oxidative dehydrogenation of ethane to ethylene is endothermic, meaning that heat energy must be supplied to drive the reaction. Similarly, propane and higher alkanes may be dehydrogenated to yield olefins.
Alkenes such as ethylene and propylene are typically higher value chemicals than their corresponding alkanes. This is true, in part, because alkenes are important feedstocks for producing various commercially useful materials such as detergents, high-octane gasolines, pharmaceutical products, plastics, synthetic rubbers and viscosity additives. Ethylene, a raw material in the production of polyethylene, is the one of the most abundantly produced chemicals in the United States. Propylene is also a raw material in a number of different processes and is utilized on a large scale. Consequently, cost-effective methods for producing these olefins are of great commercial interest.
Traditionally, the dehydrogenation of hydrocarbons has been carried out using fluid catalytic cracking (FCC), a non-oxidative dehydrogenation process, or steam cracking. Heavy alkenes, those containing five or more carbon atoms, are typically produced by FCC; in contrast, light olefins, those containing two to four carbon atoms, are typically produced by steam cracking. FCC and steam cracking have several drawbacks. First, both processes are highly endothermic, thus requiring input of energy. In addition, some of the alkane reactant is lost as carbon deposits known as coke. These carbon deposits not only decrease yields but also deactivate the catalysts used in the FCC process. The costs associated with heating, yield loss and catalyst regeneration render these processes expensive even without regard to catalyst cost.
Recently, there has been increased interest in oxidative dehydrogenation (ODH) as an alternative to FCC and steam cracking. In ODH, alkanes are dehydrogenated in the presence of an oxidant such as oxygen, typically in a short contact time reactor containing an ODH catalyst. ODH can be used, for example, to convert ethane and oxygen to ethylene and water [2]:C2H6+1/2O2→C2H4+H2O+Heat  [2].Thus, ODH provides an alternative chemical route for generating ethylene from ethane. Similarly, ODH may be used to generate higher olefins (e.g., propylene and butylenes) from corresponding alkanes. Unlike non-oxidative dehydrogenation, however, ODH is exothermic, meaning that it produces rather than requires heat energy.
Although ODH involves the use of a catalyst, which is referred to herein as an ODH catalyst, and is therefore literally a catalytic dehydrogenation, ODH is distinct from what is normally called “catalytic dehydrogenation” in that the former involves the use of an oxidant and the latter does not. ODH is attractive because, among other reasons, the capital costs for olefin production via ODH are significantly less than with the traditional processes. ODH, unlike traditional FCC and steam cracking, can be employed using simple fixed bed reactor designs and high volume throughput.
More important, however, is the fact that ODH is exothermic. The net ODH reaction can be viewed as two separate processes: an endothermic dehydrogenation of an alkane coupled with a strongly exothermic combustion of hydrogen, as depicted in [3]:
                                                                                                                                                            C                        n                                            ⁢                                              H                                                                              2                            ⁢                            n                                                    +                          2                                                                                      +                    Heat                                    →                                                                                    C                        n                                            ⁢                                              H                                                  2                          ⁢                          n                                                                                      +                                          H                      2                                                                                                                                                                                                              1                        /                        2                                            ⁢                                                                                          ⁢                                              O                        2                                                              +                                          H                      2                                                        →                                                                                    H                        2                                            ⁢                      O                                        +                    Heat                                                                                                                                            C                  n                                ⁢                                  H                                                            2                      ⁢                      n                                        +                    2                                                              +                                                1                  /                  2                                ⁢                                                                  ⁢                                  O                  2                                                      →                                                            C                  n                                ⁢                                  H                                      2                    ⁢                    n                                                              +                                                H                  2                                ⁢                O                            +              Heat                                      .                            [        3        ]            Energy savings over traditional, endothermic processes can be especially significant if the heat produced in the ODH process is recaptured and recycled. Unfortunately, although ODH offers the possibility of cost-effective olefin production, the high cost for short-chain alkane and olefin transportation has limited interest in the exploitation of remote site natural gas.